(1) Field of the Invention
The present invention relates to a soundproofing cladding panel, and to an aircraft including such a panel.
The invention is thus situated in the field of treating acoustic nuisance on board an aircraft.
(2) Description of Related Art
The present invention relates more particularly to systems for treating noise. Reducing the sound level of noise is a problem that is increasing because of the impacts of noise on people's comfort and health. This problem is encountered, particularly but not exclusively, in the technical field of aircraft, and in particular of rotary wing aircraft.
A rotary wing aircraft comprises at least one lift rotor that is driven in rotation by a drive train. The drive train may include at least one engine and at least one main gearbox (MGB) interposed between the engine and the lift rotor.
Furthermore, an aircraft may include movable mechanical member for cooling equipment on board the aircraft, and in particular the main gearbox and also electronic equipment. Movable mechanical members can also be used for delivering air to a cabin. Conventionally, a fan is used for cooling equipment and/or for moving a mass of air.
Thus, an aircraft generally has multiple sources of noise, e.g. including one or more rotors, a main gearbox, and blades of turboshaft engines or indeed of fans.
Under such circumstances, these noise sources on a rotorcraft can cover the entire spectrum of frequencies that are audible to humans. This frequency spectrum conventionally extends from audible noise having a frequency component of about 20 hertz (Hz) to audible noise having a frequency component of about 20 kilohertz (kHz).
Consequently, an aircraft may be fitted with active and/or passive devices for performing sound reduction.
In particular, a structural panel of an aircraft and/or a cladding panel of the aircraft can be designed to optimize the acoustic comfort of occupants of the aircraft. An aircraft panel can then specifically have the function of providing acoustic insulation or acoustic damping depending on the strategy followed, while also having the requisite mechanical strength.
In a first known embodiment, a cladding panel is provided with at least one wall made of composite materials. For example, it is possible to use an epoxy matrix reinforced by woven fibers.
In a first variant, the panel may comprise a monolithic wall.
In a second variant, the panel may comprise two walls sandwiching a core, e.g. a honeycomb core.
That first known embodiment thus seeks to provide a cladding panel made of composite materials.
That first known embodiment presents the advantage of making it possible to obtain a panel presenting stiffness and strength that are sufficient to comply with aircraft certification regulations, e.g. being capable of withstanding the loads to which they might be subjected in flight or during a crash.
The panel can then be a self-supporting panel suitable for carrying equipment.
The term “self-supporting” when used with a panel or a wall designates a panel or a wall presenting some minimum amount of stiffness so as to be capable of generally retaining its shape under levels of stress as predetermined by the manufacturer, such as the levels of stress to which a cladding panel is usually subjected on board an aircraft (vibration, supporting passengers, small impacts), or indeed for carrying small pieces of equipment with as few fastener points as possible.
Furthermore, the panels that are obtained are relatively light in weight, given that weight is usually an important parameter for an aircraft.
An aircraft manufacturer is then inclined towards fabricating cladding panels out of composite materials. Nevertheless, such a cladding panel made of composite materials may present a fabrication cost that is relatively high.
In a second known embodiment, a panel may have a single wall obtained using relatively inexpensive methods, such as forming or molding methods that use small amounts of polymer without any woven fibers. By way of example, the wall of a panel may be made using a component made of thermoformed polymer, or a stamped sheet, or indeed made of plastics material that has been injected into a mold.
Nevertheless, for equal weight, a panel of the second known embodiment presents stiffness that is less than that of a panel of the first known embodiment.
Making a panel of the second known embodiment that is self-supporting therefore leads to a panel that is relatively heavy. The thickness of a panel of the second known embodiment needs to be maximized in order to impart a self-supporting nature to the panel.
From an acoustic insulation point of view, a panel presents a capacity for acoustic insulation that generally complies with a “mass” relationship in the range of frequencies that have an impact on an individual's hearing comfort. Such a mass relationship specifies that the ability of a panel to provide insulation as measured in decibels varies in proportion to the logarithm of the mass per unit area of the panel.
For equal stiffness, a panel of the second known embodiment thus presents better capacity for acoustic insulation than does a panel of the first known embodiment.
In addition, the insulation capacity of a cladding panel also depends on its critical vibration frequency.
At that critical frequency, the acoustic insulation of a panel becomes smaller. It is known that the critical frequency decreases with increasing stiffness of the panel. Under such circumstances, for equal weight, a panel made of polycarbonate presents a critical frequency that is higher than the critical frequency of a panel made of composite materials.
The critical frequency of a polycarbonate panel may then lie outside the audible frequency range.
A manufacturer is then confronted with a difficult choice for optimizing the sound comfort of an occupant of an aircraft. It would be advantageous to make a cladding panel of the second known embodiment in order to move the critical frequency of the panel away from the audible frequency range. Nevertheless, such a panel runs the risk of not having sufficient mechanical strength to be self-supporting.
If the manufacturer selects the first known embodiment for the purpose of stiffening the panel, the manufacturer then obtains a panel that is more expensive and that runs the risk of having a critical frequency that is troublesome.
Fabricating a cladding panel can thus involve a compromise that is difficult to resolve in satisfactory manner.
It can be understood that an acoustic insulation panel cannot be considered as being a mere assembly of one or more conventional walls.
In order to improve acoustic insulation, and in particular in order to treat noise present at high frequencies, i.e. frequencies higher than 1000 Hz, for example, a panel may have two walls that are decoupled.
Document FR 2 939 406 suggests interposing a foam between two walls in order to decouple those two walls. The decoupling seeks to avoid transmitting vibration from a first wall that is excited by noise to a second wall.
The foam may optionally include inserts to prevent it from sagging.
The acoustic performance of the panel is then not necessarily associated with a mass relationship.
Nevertheless, it can be difficult to implement panels of the same type as the above-described second known embodiment. Decoupling the walls requires the use of walls that are thick in order to impart acceptable mechanical strength to the panel, with the corollary of being too heavy.
The first wall may comprise a sandwich structure of composite materials for imparting a self-supporting nature to the panel, with the second wall possibly being a flexible wall. Given the mass relationship, the panel must nevertheless be of considerable thickness in order to present sufficient mass to obtain good insulation.
The cost of producing such a panel may also be relatively high.
In a first strategy, a manufacturer may decide to treat noise by using an insulating panel in order to prevent the panel transmitting noise that has been transmitted thereto.
The invention seeks to provide such an insulating panel.
In a second strategy, the panel acts by damping rather than insulating, seeking to dissipate noise that has been transmitted to the panel.
Documents FR 2 894 539 and EP 0 894 617 appear to relate to damping panels.
Document FR 2 894 539 describes a panel having a porous top shell and a leakproof bottom shell provided with at least one housing. The panel then presents at least one cavity defined by the wall of a housing and the facing zone of the top shell. Noise can then pass through the top shell so as to be damped in the cavity.
Document EP 0 894 617 describes a panel having covering and/or decorative layers covering two opposite sides of a non-woven support. That support is penetrated by prism-shaped zones connected to the covering and/or decoration layers. A covering layer may be porous.
Furthermore, Document US 2011/091673 describes a body having two walls and a foam shaped in a mold. The mold thus deforms at least one of the walls and the foam.
Document GB 2 462 373 describes a covering for an automobile.